An exhaust gas purification device that is arranged in an exhaust path of an engine and eliminates nitrogen oxide (NOx) in exhaust gas (purifies the exhaust gas) using a catalyst has conventionally been used. Patent Document 1 discloses an in-line four-cylinder engine having an exhaust path, in which a pair of NOx catalysts, i.e., a first selective reduction NOx catalyst and a second selective reduction NOx catalyst are arranged in series. A first ammonia supply system is arranged upstream of the first selective reduction NOx catalyst, which is upstream of the second selective reduction NOx catalyst, and ammonia is supplied from the first ammonia supply system to the first selective reduction NOx catalyst. Ammonia is supplied from a second ammonia supply system to between the first selective reduction NOx catalyst and the second selective reduction NOx catalyst in the exhaust path.
Patent Document 2 discloses a technical idea of making the volume of a first selective reduction NOx catalyst smaller than that of a second selective reduction NOx catalyst and thereby raising the temperature of the selective reduction NOx catalysts to a desired temperature early to improve a NOx purification rate during low-temperature running.
Some of V-engines and the like adopt a configuration including a pair of exhaust paths arranged in parallel. In this configuration, a catalyst is provided for each exhaust path.